It seems that water is the preferred coolant in low temperature reactors, but what about high temperature, such as LFR or even more difficult DFR?

I guess that some noble gas would be a winner, because noble gases are inert. Helium would be the winner if we look at physical properties. But how about Argon that is highly abundant in atmosphere – Is it hopelessly inferior? With Argon coolant the heat exchanger would become larger, ten times perhaps.

This comparison between Helium and Argon is interesting because Helium is getting scarcier and pricier while Argon can be expected to stay as it is.

Transuranic heavy elements may not be used where there is life.
– Sapphire & Steel intro